Under existing medical practice, each time an implantable cardioverter defibrillator (ICD) is implanted in a human patient, an intraoperative testing procedure is attempted in order to determine a minimum defibrillation threshold (DFT) in terms of the number of joules of electrical energy required to successfully defibrillate a patient for the particular electrode lead combination which has been implanted in that patient. The intraoperative testing procedure involves inducing ventricular fibrillation in the heart and then immediately delivering a defibrillation countershock through the implanted electrode leads of a specified initial threshold energy, for example, 20 joules for a monophasic countershock. If defibrillation is successful, a recovery period is provided for the patient and the procedure is sometimes repeated one or more times using successively lower threshold energies until the defibrillation countershock is not successful or the threshold energy is lower than about 10 joules. If defibrillation is not successful, subsequent countershocks of the ICD's maximum output are immediately delivered to resuscitate the patient. After a recovery period, the procedure is repeated using a higher initial threshold energy, for example, 25 joules. It is also possible that during the recovery period prior to attempting a higher initial threshold energy, the electrophysiologist may attempt to lower the DFT for that patient by moving or changing the configuration of the electrodes.
The intraoperative testing procedure is designed to accomplish a number of objectives, including patient screening and establishing a minimum DFT for that patient. Typically, if more than 30 to 35 Joules are required for successful defibrillation with a monophasic countershock the patient is not considered to be a good candidate for an ICD. Otherwise the lowest energy countershock that results in successful defibrillation is considered to be the DFT for that patient. The use of the lowest energy possible for a defibrillation countershock is premised on the accepted guideline that a countershock which can defibrillate at a lower energy decreases the likelihood of damage to the cardiac tissue as well as provide the longest possible life for the device which has a finite amount of stored electrical energy with which to operate.
Traditionally, the intraoperative testing procedure has been accomplished by using an external test system which is connected to the implanted electrodes for delivering the test countershocks and also for delivering rescue countershocks in the event that a test countershock is unsuccessful. Once an appropriate DFT has been established for a given configuration of electrodes, the external test system is disconnected from the implanted electrodes and the actual ICD is connected to the implanted electrodes and then implanted in the patient.
Recent efforts to improve the efficiency of ICDs have led manufacturers to produce ICDs which are small enough to be implanted in the pectoral region, thereby enabling the housing of the ICD to form a subcutaneous electrode. Examples of ICDs in which the housing of the ICD serves as an optional addition electrode are described in U.S. Pat. Nos. 5,411,539 and 5,385,574. An example of a pectorally implanted ICD in which the housing is utilized as an electrode is described in U.S. Pat. No. 5,403,363. When the housing of the ICD is used as one of the electrodes, it is necessary to test that electrode as part of the intraoperative testing procedure.
Presently, all ICDs which have a housing electrode utilize some type of housing electrode emulator to perform the intraoperative testing procedure. One example of such a housing electrode emulator is disclosed in U.S. Pat. No. 5,411,539 which provides a largely reusable housing electrode emulator for screening patients for suitability for permanent implantation of such an ICD. The system disclosed in this patent has a reusable, sterilizable conductive can conforming to the dimensions of the ICD to be implanted. The reusable emulator has an electrical and mechanical attachment mechanism to connect to a standard lead and to the external test system. Once DFT testing is completed, the lead is disposed and the reusable emulator is removed and resterilized. Another example of a housing electrode emulator is disclosed in pending patent application Ser. No. 08/535,666, which describes a one piece, disposable pulse generator emulator for emulating a housing electrode of a pectorally implantable ICD. The emulator is connected by a permanently affixed lead an external test system to screen a patient for candidacy for an ICD. The one piece, disposable emulator has a housing that has substantially the same conductive geometry as does the desired ICD.
One alternative to the use of a housing electrode emulator would be to implant the ICD and utilize the ICD itself to deliver the test defibrillation countershocks for the intraoperative procedure. There are two disadvantages with this alternative. First, the use of the ICD to deliver the test defibrillation countershocks will necessarily reduce the overall energy stored in the ICD. This reduces the life of the device and decreases the total number of defibrillation countershocks which the device can deliver. Second, the delivery of any rescue shocks during the intraoperative procedure obviously cannot be accomplished using the ICD and must be initiated using an external defibrillator system. The use of an external defibrillator system is less desirable because of the larger amounts of energy required and the increased risk of tissue damage to the heart, as well as the possibility of damage to the ICD as result of delivery of a rescue shock from an external defibrillator system.
While the use of housing emulator electrodes for performing intraoperative testing prior to implantation of an ICD having a housing electrode are acceptable, the use of such housing emulator electrodes necessitates extra steps in the implantation procedure and increases the costs of the implantation procedure. Accordingly, it would be advantageous to provide an intraoperative test system for a pectorally implantable ICD which did not require the use of housing electrode emulator.